1. Field of the Invention
The present invention relates to an optical device. More particularly, the invention relates to a non-mechanical optical wavelength selective switch.
2. Description of Related Art
Fiberoptic wavelength division multiplexing (WDM) has emerged as the dominant platform for telecommunications, providing a major leap in capacity by enabling a single fiberoptic cable to transmit multiple waves of light at once thereby multiply increasing communication bandwidth. WDM systems transmit information by employing optical signals including a number of different wavelengths, known as carrier signals or channels. Each carrier signal is modulated by one or more information signals. For further bandwidth expansion, intelligent optical networks become critical in which optical channels are dynamically routed/switched in the optical layer. Therefore, wavelength selective optical routers/switches are a key component in next-generation optical networks. Such devices are analogous to electrical switches in electrical networks. Optical wavelength selective switches can be used to perform basic WDM functionalities, such as optical signal routing, channel add/drop, and dynamic multiplexing/demultiplexing. However, optical wavelength selective switching has not been widely adopted because of the lack of commercially available components of needed reliability.
In an optical switch, a light signal must accurately enter into an optical fiber or much of the signal strength is lost. The alignment requirements of micro-optic devices are particularly stringent, as fiber core diameters are typically as small as 2 to 10 micrometers and their acceptance angle is fairly narrow. Furthermore, insertion losses reduce the amplitude of the optical signal. Therefore, optical switches that accept light from an input optical fiber and selectively couple that light to any of a number of output optical fibers must transfer that light precisely and within a small acceptance angle for the light to efficiently enter the fiber. Currently, optical wavelength selective switching is achieved by coupling optical filters with mechanical optical switches. Consequently, such devices have many drawbacks including slow switching speed, low reliability, and bulky size. One such mechanical wavelength selective switch is described by Lee in U.S. Pat. No. 6,192,174 issued on Feb. 20, 2001. It is therefore greatly desirable to have integrated optical wavelength selective switches that direct light beams according to their wavelength without moving parts, a feature generally associated with high reliability and high speed.
A non-mechanical optical wavelength selective switch is described and claimed by Wu et al. in U.S. Pat. No. 5,694,233 issued on Dec. 2, 1997. FIG. 1 depicts the optical wavelength switch 999 from Wu et al., herein incorporated by reference. A WDM signal 500 containing two different channels 501, 502 enters the optical wavelength switch 999 at an input port. A first birefringent element 30 spatially separates the WDM signal 500 into horizontal and vertically polarized signals 101 and 102 via a horizontal walk-off element. Signals 101 and 102 are coupled to a two-aperture polarization rotator 40. The polarization rotator 40 selectively rotates the polarization state of either signal 101 or 102 by a predefined amount to render their polarization parallel. The polarization rotator 40 consists of two sub-element rotators that form a complementary state so that when one aperture turns ON the other turns OFF. By way of example, one signal 102 in FIG. 1 is rotated by 90xc2x0 so that signals 103, 104 exiting the polarization rotator 40 are both horizontally polarized when they enter a wavelength filter 61.
A waveplate wavelength filter 61 selectively rotates the polarization of wavelengths in either the first or second channel to produce filtered signals 105 and 106. For example, the wavelength filter 61 may rotate wavelengths in the first channel 501 by 90xc2x0 but not wavelengths in the second channel 502. The filtered signals 105 and 106 then enter a second birefringent element 50 that vertically walks off the first channel into beams 107and 108 and the second channel into beams 109 and 110. A second wavelength filter 62 then selectively rotates the polarization of signals 107 and 108 but not signals 109 and 110 thereby producing signals 111, 112, 113 and 114 having polarizations that are parallel to each other. A second polarization rotator 41 then rotates the polarizations of signals 111 and 113, but not 112 and 114. The resulting signals 115, 116, 117, and 118 then enter a third birefringent element 70. This birefringent element 70 combines signals 115 and 116, into the first channel, which is coupled to one output port and also combines signals 117 and 118 into the second channel, which is coupled into another output port.
As described above, by suitably controlling the polarization rotation induced by the polarization rotators 40 and 41, the optical wavelength switch 999 operates as a wavelength selective device. Furthermore, the optical wavelength switch 999 can also operate as a passive interleaver multiplexer or de-multiplexer via a fixed set of polarization rotators in 40 and 41.
The optical wavelength switch 999 has major drawbacks. First, it is disadvantageously based on a large spatial separation between two fibers located on the same side. The configuration requires individual imaging lens for each fiber port and consequently requires large and long-length crystals to deflect the beams. The use of three separated collimators to couple the signals into and out of optical fibers adds size, complexity, and cost. Moreover, the long couple distance increases signal loss. The bulky size also leads to instability, since operational stability is inversely related to the mass of birefringent materials. As a result, the optical wavelength switch 999 typically has high loss, excessively large size, and is expensive to produce and less stable in operation. Second, the electrically controllable polarization rotators 40 and 41 are based on a two-part aperture design that rotates the optical beams separately in a complementary manner, i.e. when one turns ON the other turns OFF. Such a design is primarily for the incorporation of organic liquid crystal device (LCD) based polarization rotators. The LCD usually employs surface electrodes in the light path to apply an electrical field. Consequently, two individually controllable rotators can be easily fabricated on the same element via electrode patterns. However, the use of liquid crystal materials leads to undesirable properties of slow speed and large temperature dependence, which are objectionable for optical network applications. Recent progress in inorganic magneto-optic and electro-optic materials has opened new opportunities to produce solid-state optical switches of faster speed and high stability. However, the two-part separately controlled polarization rotator 40, 41 in the optical wavelength switch 999 is unsuitable for incorporating inorganic crystals. This is so because it is very difficult and impractical to apply two opposite fields with reasonable uniformity to two adjacent Faraday crystals or electro-optic crystals, due to the strong field interference across the small spatial separation.
An optical interleaver described by Li in U.S. Pat. No. 6,212,313 issued on Apr. 3, 2001 represents some improvement by using dual fiber sharing a single imaging lens to reduce the size of the optical device. However, wavelength selective devices based on Li are primarily designed for passive interleaver applications. Li is not amenable to active wavelength selective switches, because it too is based on the same two-part aperture polarization rotator design described by Wu. For reasons described above, the Li invention is unsuitable for wavelength switching/routing applications using solid-state materials of magneto-optic garnet or electro-optic crystals as the controllable polarization rotators. Moreover, reflection type optical configurations like Li are based on the use of either three separated collimators or a triple collimator on one side to couple the signals into and out of optical fibers. The use of multiple individual collimators significantly increases size and adds cost. Also, a triple collimator substantially increases complexity, resulting in increased interdependency among alignments of elements along each optical path. Therefore, the manufacture of Li type devices is difficult and production costs are high.
Due to the difficulties discussed above, solid-state wavelength switches are not commercially viable. Therefore, there is a need for an improved optical wavelength switch that overcomes the deficiencies inherent to the related art. It would be particularly desirable to provide optical wavelength selective switches combining low optical insertion loss, high-speed switchability, and high reliability. It is also important that these switches are constituted from components of small size, require a reduced number of alignment steps, and have large assembly tolerance to facilitate low-cost manufacture. The inventive optical devices described here provide these critical attributes.
The present invention provides a compact, robust and economical non-mechanical optical wavelength selective switch that can be efficiently coupled to optical fibers using fewer parts and having larger assembly tolerance than the prior art. The inventive three-port device divides the incoming WDM optical signals into two subsets of channels and switchably directs them into two selected output ports in response to an electrical control signal. The invention allows for the use of inorganic crystal material to achieve fast, reliable and stable wavelength switching and filtering functions. The inventive wavelength selective switch uses at least one single lens to couple two fibers achieving small beam separation thus small size and low material cost. The invention further consists of a light-bending device, situated to compensate for the angle between the two light beams that share the same lens, advantageously increasing alignment tolerance.
The solid-state optical wavelength selective switch of the present invention has several advantages over the related arts. First, the inventive configuration places two fiber ports on the same side to be physically close and adjacent to each other and to share the same imaging element, leading to fewer optical elements comprising the entirety of the switch. The closely spaced beam propagation arrangement reduces the size requirements for each birefringent beam deflection element, consequently lowering material costs. The design also results in a smaller footprint as compared to the prior art. Prior non-mechanical optical wavelength switches have an arrangement wherein each optical port has its own individual imaging element, disadvantageously requiring larger dimensions, and hence greater volume, within each separate component comprising the device. Second, the present invention incorporates a beam angle correction system, allowing adjustment of position and angle substantially independently, reducing position sensitivity and achieving maximum light coupling. This inventive configuration greatly reduces assembly and packaging complexity and, therefore, is particularly desirable for volume production. Third, the present invention is based on electrically controllable polarization rotators having a single-part aperture. This simple configuration is better suited for using magneto-optic Faraday crystals or inorganic electro-optic materials as the controllable polarization rotator. Prior non-mechanical optical wavelength switches have disadvantageous configurations wherein the controllable polarization rotators utilize a two-part aperture of different rotations that is not amenable with using inorganic polarization rotating materials.
In one aspect of the present invention, an optical signal within different channels may be rapidly and reliably switched between two optical paths, according to applied electrical control signals. The inventive optical wavelength switch may be used in telecommunications systems/sub-systems for applications such as WDM channel add/drop, dynamic reconfiguration, multiplexers/demultiplexers, and signal routing. The inventive optical wavelength selective switches are particularly suited for WDM optical network applications, where high-speed and reliable switching is required. These and other advantages of the inventive optical switches are elaborated in the specific embodiments described herein.
The wavelength switch described here is a polarization-rotation based device in which a randomly polarized input light beam is split into a pair of beams of two orthogonal polarizations. The optical wavelength is further split into two sets of complementary spectra of different polarizations by passing through waveplate-based filters. The light beams from one spectrum go to one fiber but that with the other spectrum goes into another fiber. The electrically controlled polarization rotators switch the state of polarization of the light beams from one to the other, consequently switching the two sets of wavelengths from one port to another port. The inventive device advantageously achieves routing while conserving all optical energy regardless of the polarization of the input signals.